Automatic darkening liquid crystal protective shields, also known as automatic darkening filters, or ADFs, are often constructed from a combination of polarizing filters and layers of liquid crystals. Examples of such filters are described in U.S. Pat. Nos. 6,097,451 and 5,825,441, both to Hörnell and Palmer.
The ADFs change from a light (transparent) state to a darker (nearly opaque) state in response to a control signal. For applications like welding where protection from intense levels of incident light is desired, the filters are normally mounted in a facemask, and the control signal is initiated by incident light from the welding arc. The incident light strikes a detector on the welding helmet which in turn sends a signal to the ADF. In this way, the filter is clear or transparent when not subjected to the glare of the welding arc and darkens immediately when exposed to such glare. This enables a welder to perform a welding operation and also to perform tasks outside the welding area without removing the protective shield.
Conventional filters have one particular drawback, namely, the filter effect of the liquid crystal layers is angularly dependent. In other words, the viewing area as seen by the user may appear to darken unevenly. Variations in cell gap and unwanted birefringence in the adhesive layers or the polarizers may also reduce homogeneity, resulting in variations in shade over the viewing area of the welding filter.